In fuel injection systems, of self-igniting engines in particular, the fuel quantities injected by injectors into the combustion chambers are divided into a plurality of partial injections. The partial injections usually follow one another in a rapid succession and may include one or more pilot injection(s) applied before a main injection. The time interval between two partial injections is implemented by the pause time between two electric trigger pulses of the injectors. The partial injections make improved mixture preparation and thus lower exhaust gas emissions of the engine, lower noise development during combustion, and higher mechanical power output of the engine possible.
In the case of the above-mentioned partial injections, the accuracy of the injected quantities is of great importance. However, each injection causes a brief drop in the fuel pressure in a fuel line connecting a high-pressure accumulator, known as a rail, to the corresponding injector. Such a pressure drop results in a fuel pressure wave between the rail and the injector after the end of the injector triggering; the effect of this wave on the injected quantity of the subsequent partial injections diminishes with an increasing time interval between the particular successive injections. This pressure wave effect intensifies with increasing lift frequency of the nozzle needle of the injector, so that taking it into account, also in future injector systems in particular, in which high-speed piezoelectric actuators are used as injection actuators for nozzle needle control in the particular injector, becomes increasingly important.
Since the above-described pressure wave phenomenon is of a highly systematic nature, and although it essentially depends on the time interval between the corresponding injection(s), the injected fuel quantity, the hydraulic fuel pressure, and the fuel temperature in the rail, compensation via an appropriate control function in the engine control unit may be implemented. In a method described in German Patent Application No. DE 101 23 035 for minimizing the pressure wave effect, the effect on the injected quantity of the particular injector is measured and the results of this measurement are taken into account in presetting the control data of the injector, specifically based on a previously empirically, i.e., experimentally, determined fuel quantity wave as a function of the time interval between the partial injections involved. The measured effect of the quantity on a subsequent injection is stored in characteristic maps, and the effect of the quantity is then compensated during the operation of the engine by appropriately modifying the duration of the energized state of the actuator which effects the subsequent injection.
The characteristic map is filled with data experimentally by measurements on a hydraulic test bench. The quantities influenced are ascertained in the form of “quantity waves” as a function of the interval between the corresponding injections and used for filling the characteristic map with the aid of a special algorithm. The excess or reduced quantities thus ascertained are stored in the above-mentioned characteristic maps and compensated during the operation of a control program of the engine by making the appropriate deductions in a quantity path of the engine control.
In the above-mentioned pressure wave correction, in principle a number of input and output quantities must be taken into account, the exact relationship between these quantities being extremely complex, since there are mutual dependencies such as interactions between the input quantities in particular. For this reason, considerable simplifications are necessary in the pressure wave correction to map the pressure wave phenomenon using the fewest possible characteristics maps; therefore, when mapping the pressure wave system, a considerable portion of the correction accuracy that would be possible in principle is lost.
It is therefore desirable to improve a method of the type mentioned above in such a way that a more accurate pressure wave correction than in the related art is made possible, which takes into account the largest possible number of input and/or output quantities in the pressure wave correction, omitting the fewest possible factors considered negligible, while using the least possible technical complexity at the same time.